Method of forming seamed metal tube

ABSTRACT

The method of forming a seamed metal tube having a metal coating of this invention includes galvanizing the strip prior to welding. The strip is then formed into an open seam tube and welded in an inert atmosphere with the seam located in the lower portion of the tube. Finally, the metal coating is caused to flow downwardly over the seam, coating the seam by several means. In one embodiment, the strip is preformed into an arcuate shape and galvanized, wherein the zinc coating in creases in thickness toward the lateral strip edges, such that the coating will flow over the seam following welding. The tube may also be reheated following welding, preferably in an enclosure containing an inert atmosphere which includes the weld apparatus. In an other embodiment, an inert gas is directed over the inner and outer surfaces of the tube, adjacent the seam, driving the molten metal downwardly over the seam to coat the seam. In another embodiment, the lateral edges of the strip are formed with indentions or grooves directed toward the lateral edges which direct the molten metal downwardly over the seam.

This invention relates to an improved method of forming a seamed metaltube having a metal coating. More particularly, the present inventionrelates to a continuous or in-line process of forming a ferrous metaltube coated with a protective metal coating, preferably, zinc oraluminum.

BACKGROUND OF THE INVENTION

Methods of continuously or in-line forming of a seamed steel tube from acontinuous strip or skelp are well known. In a conventional tube formingmill, the continuous strip is first cleaned and conditioned, then rolledto form an open seam tube having nearly abutting edges at the top of thetube. The edges are then welded together by one of several conventionalmethods which generally include heating the edges and then eitherforging the edges together with squeeze rolls and/or flux welding theseam. The edges of the tube may be heated, for example by resistancewelding, electric arc or by high frequency induction welding. Highfrequency induction welding is a form of electric resistance welding,wherein the open seam tube is received through an electric work coilwhich creates a strong magnetic field, which in turn induces a currentto flow around the tube and in the "Vee" formed as the edges of thestrip are welded. An impeder is generally located within the tube, whichforces the current down the nearly abutting edges of the open seam tube,heating the tube edges to a hot forging temperature. The tube edges arethen forged by squeeze rolls which drive the molten edges together toform an integral seam.

In-line galvanizing and coating or painting processes are also wellknown. The strip or skelp may be galvanized or painted on one or bothsides prior to forming and welding, or the welded seamed tube maygalvanized by immersing the tube in a molten zinc bath. Where the stripis coated with a protective coating prior to seam welding, the coatingwill burn off or melt in the seam zone because the welding operationinvolves the melting of the tube material, which is generally steel.Thus, the temperature at the seam may be 2,300° F., or greater. Wherethe strip is coated with a metal, such as zinc or aluminum, the metalwill melt during welding and flow downwardly away from the seam, whichis located at the top of the tube. A zinc coating solution has also beenused to paint the exterior surface of the seam. However, such coatingshave poor adherence and are mainly cosmetic. The failure of the presentprocesses to fully coat and thus protect the tube seam is evident by thefact that the weld area is generally the first to fail in acceleratedcorrosion tests. At present, there is no commercial in line processcoating the entire internal and external surfaces of a tube with zinc.Thus, there has been a long-felt need to provide an improved coatingprocess, particularly on the seam.

The continuous tube forming process and apparatus of this inventionsolves the above identified problems and produces a superior metalcoated tube. The process of this invention assures a fully coated weldedseam without substantial additional costs.

SUMMARY OF THE INVENTION

As set forth above, the present invention relates to an improved tubeforming and coating process and apparatus. The method of forming aseamed metal tube of this invention is particularly, but not exclusivelyadapted to a continuous process wherein the strip is coated on one orboth surfaces with a metal coating prior to forming and welding. Thus,the method includes first coating one or both surfaces of a metal stripor skelp with a metal coating, preferably a coating of zinc, aluminum orother alloys. The process then includes rolling and forming the stripinto a tube-shaped strip or open seam tube having opposed spaced nearlyabutting lateral edges in a lower portion of the open seam tube. Theprocess then includes heating and integrally welding the adjacent edgesof the strip to form a tube having a welded seam in the lower portion ofthe tube. The most preferred method includes inductively heating theopposed lateral edges of the strip by moving the strip continuouslythrough an induction coil with the nearly abutting edges orientedgenerally downwardly and then forging the edges together with squeezerolls to form an integrally seamed tube having a welded seam orienteddownwardly.

The method of this invention may then include reheating at least a lowerportion of the tube to the melting temperature of the metal coating,such that the molten metal coating flows downwardly and coats the seam.The metal coating on the inner surface of the tube flows downwardly andaccumulates over the seam. In a most preferred embodiment, the open seamtube is welded in a substantially inert atmosphere and the seamed tubeis reheated in an inert atmosphere to cause the metal coating to flowdownwardly over the seam prior to oxidation of the molten seam. This isaccomplished in the disclosed embodiment by enclosing the tube weldingand reheating apparatus in an enclosure and injecting nitrogen or otherinert gas under pressure into the enclosure to maintain an inertatmosphere. Where the tube is externally coated with a metal coating,the tube seam is preferably scarfed before reheating.

A preferred embodiment of a method of this invention includes coatingboth surfaces of the strip with zinc, prior to forming. In a mostpreferred method, the strip is partially formed into an arcuate shape,prior to galvanizing, wherein the thickness of the zinc coating iscontrolled to form a coating having a thickness which increases from amid-portion toward the lateral edges of the arcuate strip. It will beunderstood, however, that the edges should be free of coating to permitwelding. Thus, the metal coating is removed from the edges followinggalvanizing by edge conditioning comprising either slitting the edges orotherwise removing the coating at the edges. Where the thickness of themetal coating increases toward the edges, it is possible to obtain areflow of the metal coating over the seam, without reheating followingwelding, particularly where the edges are welded in an inert atmosphere.

Other advantages and meritorious features of the continuous tube formingand coating process of this invention will be more fully understood fromthe following description the preferred embodiments, the claims, and thedrawings, a brief description of which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram illustrating a preferred embodimentof the continuous tube forming and coating process of this invention;

FIG. 2 is a cross-sectional view of the strip following preforming inthe direction of view arrows 2--2 of FIG. 1;

FIG. 3 is an end view of a gas shaping nozzle at the outlet of thegalvanizing tank as shown in FIG. 1, in the direction of view arrows3--3;

FIG. 3A is an end cross-sectional view of a strip following endconditioning as shown in FIG. 1, in the direction of view arrows 3A--3A;

FIG. 4 is a cross-section view of a tube-shaped strip or open seam tubeformed by the process of FIG. 1, in the direction of view arrows 4--4;

FIG. 5 is a side cross-sectional view of the shaping nozzle shown inFIG. 3;

FIG. 6 is a side elevation partially broken away of a tube entering theseam welding station having an internal seal;

FIG. 7 is a side elevation partially cross-sectioned of an inductionwelding apparatus;

FIG. 8 is an end view of the tube forming step by squeeze rolls;

FIG. 9 is a partially cross-sectioned view of a scarfing apparatus inthe inert gas chamber of FIG. 1;

FIG. 10 is a partially schematic, partially cross-sectioned side view ofanother embodiment of a tube welding and seam coating apparatus whichmay be used with the process of this invention;

FIG. 11 is an end cross-sectional view of the partially formed strip inan optional edge conditioning apparatus which rolls a pattern on thestrip;

FIG. 12 is an end cross-sectional view of the apparatus shown in FIG.10, in the direction of view arrows 12--12;

FIG. 13 is a top view of a gas assist seam coating apparatus;

FIG. 14 is a side cross-sectional view of an induction welding impederwhich may be utilized in the process of this invention;

FIG. 15 is an end cross-sectional view of the impeder shown in FIG. 14,in the direction of view arrows 14--14;

FIG. 16 is an end view of the impeder shown in FIG. 14;

FIG. 17 is an end cross-sectional view of the impeder shown in FIG. 14in the direction of view arrows 16--16;

FIG. 18 is a partial end cross-sectional view of one lateral edge of thestrip following edge conditioning;

FIG. 19 is a partial end cross-sectional view of a second embodiment ofthe lateral edge of the strip following edge conditioning;

FIG. 20 is a partial end cross-sectional view of a seamed tube formedfrom a strip which was edge conditioned as shown in FIG. 18; and

FIG. 21 is a partial end cross-sectional view of the seamed tube shownin FIG. 20 following reflow of the metal coating over the seam.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in the schematic flow diagram of FIG. 1 and described above,the tube forming and coating and process of this invention isparticularly, but not exclusively adapted for processing of endlesslengths of untreated strip steel or skelp, such is normally processed bya continuous tube forming mill. Improvements described herein can also,however, be used with pre-coated strip, wherein a metal coating isapplied to one or both surfaces of the strip prior to processing.Further, the improvements described herein can also be used in a batchor non-continuous process. The process of this invention will, however,be described in relation to the continuous process disclosed in FIG. 1.

Metal strip or skelp, preferably strip steel, 20 is supplied to the tubeforming mill in coils 22, which are mounted on a pay-out reel 24. Thecoil is mounted for free rotation on the reel 24 as required by ancontinuous tube forming mill. As will be understood by those skilled inthe art, the strip is processed by the mill substantially continuouslyat a constant rate. The advancement of the flat strip or skelp 20through the mill is effected primarily by engagement between the stripand the forming and sizing rolls which rotate at a relatively constantspeed. The strip 20 is thus drawn into the mill from the pay-out reel24.

Because the length of each coil 22 of strip steel is taken up by themill in a relatively short time, means must be provided for splicing theend of one coil to the next, which is accomplished at the splicingstation 26. In the splicing station 26, the end of the coil beingprocessed in the mill is sheared or cut to provide a square end and theend is then welded to the beginning of the next reel. A conventionalaccumulator 28 in the form of a loop of strip is provided havingsufficient length to continuously feed the strip to the mill while thetrailing end of the coil is held stationary for shearing and welding.The loop may be formed by feeding the strip over a series of rolls (notshown) which are mounted to freely rotate as is well-known in the art.When the splice is complete, the strip is again paid-out over theaccumulator rolls for the next splicing operation when the next reel isexhausted.

Because the coil of skelp is received by the mill normally includes oiland may include other contaminants, it is generally necessary to cleanand prepare the strip prior to coating, which in the disclosed processis accomplished at the cleaning station 30. In a typical application,the strip is cleaned and prepared by alternate alkaline and phosphatewashes, with intermediate thorough water rinses. In the disclosedprocess, the strip is also dried. The cleaned strip is now ready forgalvanizing.

In one preferred embodiment of the process of this invention, the stripis then advanced to a preform station 32, having a plurality of rolls 34which form the strip into an arcuate shape as shown at 20a in FIG. 2. Asdescribed, for example, in U.S. Pat. No. 3,696,503 of Krengle et al. thestrip is formed by a series of forming rolls which have a configurationand number sufficient to form the strip into an arcuate shape, includingrolls which engage the upper surface of the strip having a slightlyconvex shape and lower rolls which have a slightly concave shape tobegin the deformation of the strip into the desired shape. The strip isthen formed by a series of opposed rolls, each having a greater radiusof curvature than the prior rolls until the strip is formed into thepreferred arcuate shape 20a of FIG. 2. By forming the strip in themanner shown in FIG. 2, the amount of zinc which will adhere to thesurface will be that necessary to obtain a coating that will be stronglybonded to the surface of the strip having a desired thickness, goodcoverage and a good appearance.

The arcuate shaped strip 20a is then advanced through the galvanizingtank 34 and immersed in molten zinc in a convention manner as described,for example, in the above-referenced U.S. patent of Krengel, et al. Thezinc coating on the arcuately shaped strip flows downwardly toward thelateral edges of the strip, such that the thickness of the zinc coatingincreases from the mid-portion toward the edges. In a most preferredmethod of this invention, the zinc coating is further shaped by an inertgas shaping nozzle 36 located adjacent the outlet of the galvanizingtank. A suitable embodiment of a gas shaping nozzle 36 is shown in FIGS.3 and 5. As shown, the nozzle includes an axial and radially extendinggas chamber 38, including a generally cylindrical inlet portion 40 and aconical portion 42 defined by converging conical walls. A gas, such asnitrogen, is introduced under pressure through inlet ports 44 into theinlet 40 of the gas nozzle. The gas accelerates through the outletportion 38 and exits the restricted openings 46 above and below thearcuately shaped strip 20a. As shown in FIG. 3, the exit opening 46adjacent the mid portion of the strip 20a is narrower than the openingadjacent the lateral edges 48 of the strip. The liquid zinc is thusdirected and driven outwardly and downwardly toward the lateral edges48, shaping the zinc coating on the strip, such that the thickness ofthe coating increases from the mid portion toward the lateral edges 48.Baffles 50 may be employed to control the velocity of the gas exitingthe nozzle at locations along the width of the strip. As describedbelow, the purpose of shaping the zinc coating on the strip is to assurethat there is sufficient volume of coating to reflow over the seamfollowing welding, coating and protecting the seam. The requirement forshaping the coating disclosed herein will depend upon several factors,.including the speed of the line, the thickness of the zinc coating, thetemperature of the strip leaving the galvanizing bath, the shape of thestrip, etc. As will be understood by those skilled in the art, shapingthe coating as described herein will reduce the overall thickness of thecoating; however, it may not be required to preform the strip or shapethe coating in all applications, particularly where a thicker metalcoating is used. Further, it is possible to shape the coating with thegas shaping nozzle of this invention without preforming the strip intoan arcuate shape.

In a conventional mill, the strip 20 as received on the reel has a widthwhich is slightly greater than the width required to form the tube, suchthat an edge is available on each side of the strip for proper sizing ofthe tube and to provide freshly cut metal at the abutting edges formingthe seam. The side edges of the strip will be coated with zinc in thegalvanizing tank 34. Thus, it is preferable to remove the zinc from theside edges prior to welding, particularly where the edges of the openseam tube are joined by induction welding. A conventional edge shaver orslitter may be used in the edge conditioning station 54, which may beused to cut a square edge. More preferably, a chamfered edge is providedto provide relief for the forged upset and assure good welding contactbetween the edges in the welding operation described below. FIG. 3aillustrates the strip following edge conditioning, wherein the zinccoating has been removed from the lateral edges 48 of the arcuate stripand the zinc coating 56 increases in thickness from the mid-portiontoward the lateral edges 48.

The arcuate metal strip 20a is then rolled into a tube-shaped strip oropen seam tube 20b having adjacent or nearly abutting or lateral edgesat the forming station 58. The metal strip is progressively formed intoan open seam tube as it passes between rolls 60. The rolls arerotatively supported on vertical and horizontal axles (not shown) in aconventional manner. However, in the continuous tube forming and coatingprocess of this invention, the lateral edges of the strip are deformedor bent downwardly and inwardly toward one another as the tube isformed, rather than upwardly as in a conventional tube forming mill. Thelateral edges of the strip are then rolled into nearly abutting relationat the lower portion of the open seam tube 20b: however, the adjacentlateral edges are slightly spaced, as shown in FIG. 4. The open seamtube 20b is then received in the tube welding station 62, where thelateral edges of the strip are welded, as now described.

The preferred embodiment of the tube welding apparatus of this inventionutilizes high frequency induction to heat the opposed lateral edges ofthe open seam tube 20b. As shown in FIGS. 1 and 7, the induction weldingapparatus includes a work coil 64 which is connected to a source of highfrequency alternating current. The work coil 64 creates a strongmagnetic field, which in turn induces current in the open seam tubeadjacent the work coil. An impeder 66 is located within the open seamtube 20b. The impeder 66 includes a support or bracket portion 76 whichextends downwardly between the opposed adjacent lateral edges 48 of theopen seam tube 20b. A conventional induction welding impeder consists ofa non-metallic tube surrounding one or more ferrite rods. Water or millcoolant is circulated over and past the ferrite rods to remove the heatproduced by magnetic hysteresis and eddy current losses. At thefrequencies used for induction welding (200 to 800 kHz). current flowsaround the tube and along the "Vee" formed by the approaching edges ofthe strip, heating the edges to a hot forging temperature, whereby theedges are at least partially melted. The edges are then forged togetherby squeeze rolls 68 as shown in FIG. 8, forming an integral seam 71.Where the strip is steel, the temperature of the edges will be about2,300° F., or greater. The seamed tube then passes over a scarfing tool70 which removes the flash 72 from the outer portion of the seam, asshown in FIGS. 1 and 9. A back-up roller 74 engages the opposed surfaceof the tube, counteracting the pressure of the scarfing tool 70.

In the preferred tube welding and coating process of this invention, theedges of the open seamed tube are welded in a substantially inertatmosphere. In the embodiment of the tube welding station 62 shown inFIG. 1, the welding apparatus is enclosed within an enclosure 78. Theopen seam tube 20b is received in the enclosure through an inert gasseal 80 best shown in FIG. 6. The inert gas seal includes an inner plug82 which may be formed of a friction resistant thermoset plastic or aceramic, such as reinforced nylon, which receives the open seam tube20b, as shown in FIG. 6. The body portion 84 of the plug is generallycylindrical having an outside diameter which is nearly equal to theinside diameter of the open seam tube, to be closely received within thetube. The plug 82 includes a relatively thin radial support portion 86which is received between the edges of the open seam tube. A gas portextends through the support portion 86 into the plug body portion 84having an outlet 90 which injects an inert gas, preferably nitrogen,into the open seam tube. Line 92 (see FIG. 1) connects a source of inertgas 94 to the inlet in the support portion 86 of the gas plug. A fiber,ceramic or plastic outer seal 96 closely receives the outer surface ofthe open seam tube 20b, providing a seal for the open seam tube as itenters the inert atmosphere in the enclosure 78. Nitrogen gas is alsoinjected through line 98 into the enclosure 78 to produce asubstantially inert atmosphere in the enclosure. The flash 72 scarfedfrom the tube is received in a tube 100 which extends out of theenclosure 78 as shown in FIG. 9. Inert gas is injected into the tubethrough line 102 to maintain the inert atmosphere in the enclosure 78 asthe flash is scarfed from the tube. Finally, as described more fullyhereinbelow, inert gas is also injected into the impeder through line104, which is connected to the bracket 76 of the impeder. In thedisclosed embodiment, the tube is dried and cleaned prior to receipt inthe inert atmosphere enclosure 78 by an air blower 106 which blasts warmair into the open seam tube 20b. The blower dries the tube and blowssmall debris out of the tube through the spaced lateral edges 48.

As described above, the metal coating on the tube will melt or bum offin the weld zone by the forging temperature induced by the work coil 64.Further, the weld seam is relatively rough and therefore difficult tocoat. Where the tube is galvanized after welding, the zinc coating willtend to pull away from the seam as the tube emerges from the zinc bathbecause the seam is normally located at the top of the tube. In the tubewelding and coating process of the present invention, however, thenearly abutting edges 48 are located near the bottom of the tube asshown in FIG. 4, such that the zinc melted by the welding process willflow downwardly over the seam, re-coating the seam with molten zinc.Where the thickness of the zinc coating is built up adjacent the seam,the zinc melted in the welding process may be sufficient to fullyre-coat the seam without reheating, depending upon the thickness of thecoating, the diameter of the tube and the speed of the line. In otherapplications, however, it will be necessary to remelt the metal coatingto cause the metal coating to flow downwardly over the seam. Thus, thetube welding apparatus 62 in FIG. 1 includes an induction coil 110 which"reheats" at least a lower portion of the seamed tube 20c, melting thezinc at least adjacent the seam. Because the seam is located in thelower portion of the tube, the molten zinc then flows downwardly overthe inner and outer surfaces of the seam and collects at the bottom ofthe tube, coating the seam 70 with a protective zinc coating asdisclosed herein.

As used herein, reheating may comprise maintaining the temperature ofthe welded tube 20c or supplemental heating a lower portion of the tubeto a temperature above the melting temperature of the metal coatingwhich is preferably zinc. The open seam tube 20b may also be heatedprior to forging by an induction coil without an impeder. This is a lessefficient means of heating the lateral edges 48 of the open seamed tube,but rill result in heating the lower portion of the tube spaced from thelateral edges to a temperature above the melting temperature of themetal coating. Depending upon the diameter of the tube and the speed ofthe line, no further heating may be required to cause the metal coatingto flow over the seam. Alternatively, the welding temperature may bemaintained by a parallel induction coil as shown, for example, at 160 inFIG. 10. The seam, however, may be located anywhere in the lower half ofthe tube, provided the induction coil 110 is located adjacent the seam.More preferably, the seam is located a lower third of the tube. Theseamed tube 20c then exits the inert atmosphere chamber 78 through seal112, which may be a inert gas seal as described herein.

The tube is then cooled by immersing the tube in cold water or othercoolant in the cooling chamber 114, freezing the metal coating on thetube. Inert gas is prevented from escaping the tube in the weldingapparatus 62 by a cylindrical plug 114 which may be attached to the endof the impeder by a tether 116. The plug 114 may be made of a frictionresistant thermoset plastic, such as nylon, and is preferably locatedsufficiently downstream from the conduction coil 110 and followingcooling to avoid damage to the plug.

Following cooling, the tube 20c enters the final sizing station 118which includes a plurality of sizing and straightening rolls 120.Thereafter, the tube is flooded with an aqueous chromate solution andrinsed at 122, which cleans the tube prior to final finishing. In aconventional tube mill, the tube is then marked at 124 with productidentification and further markings as may be specified by the customer.A clear lacquer coat or other protective coating may then be applied inthe OD paint station 126. An induction heating coil 128 may then be usedto dry the tube and the tube is finally cut to length at the cut-offstation 130.

FIGS. 10, 12 and 13 illustrate an alternative embodiment for theinduction welding apparatus which may be used to further improve there-coating of the welded seam. Although not shown, it would beunderstood that the induction welding apparatus is preferably enclosedin an inert atmosphere, as disclosed above in regard to FIG. 1. Theinert atmosphere in and around the tube reduces oxidation of the seamand thus improves the metal coating. An impeder 140 is located withinthe open seam tube 20b having a bracket portion 142 which extendsdownwardly between the adjacent edges of the strip, as described above.An inert gas, preferably nitrogen, is fed through line 146 to thebracket portion 142 of the impeder from a source of non-oxidizing gasunder pressure 148. As described below in regard to FIGS. 13-16, thenitrogen gas is transmitted through the impeder 140 to the free end of150 of the impeder where it exits the impeder and floods the weld zonewhere the molten edges of the open seam tube are forged and weldedtogether by squeeze rolls 152. In the disclosed embodiment, the innersurface of the seam is rolled, smoothed and flattened by roller assembly154. Roller assembly includes upper and lower cylindrical rolls 156 and158, respectively, which are biased against the upper and lower surfacesof the tube to roll and flatten the inner surface of the seam 70.

The seamed tube 20c is then reheated to the melting temperature of themetal coating by induction coil 160. As described above, the inductioncoil 160 is located adjacent the seam and heats the tube to atemperature sufficient to melt the metal coating, causing the metalcoating flow downwardly over the inner and outer surfaces of the seamand coating the seam. In the most preferred embodiment where the tube isgalvanized, the tube adjacent the seam is heated to at least 420° C. Inthe embodiment of the apparatus shown in FIGS. 10, 12 and 13, an inertgas, preferably nitrogen, is also directed over the tube 20c toward theseam, driving and directing molten metal downwardly over the seam. Anitrogen manifold 162 is supported within the tube on rollers or wheels164 as best shown in FIG. 12. The manifold 162 includes a plurality ofgenerally downwardly directed openings 166 which direct nitrogen gasunder pressure downwardly toward the seam 70 as shown by arrows 166. Asfurther shown in FIG. 12, the induction coil 160 is located below apron170 which protects the induction coil and guides inert gas over theexternal surface of the tube. In the disclosed embodiment, a pluralityof staggered generally downwardly projecting nozzles are connected tonitrogen manifold tubes 174, as shown in FIG. 13. The nozzles maypreferably be staggered to avoid opposed streams of gas; however, thenozzles may also be opposed. In the disclosed embodiment, the inductioncoil 160 is located in apron 170 which further includes staggered ports178 which direct the inert gas upwardly against the lower surface of thetube, forming a sheet of inert gas which drives molten zinc downwardlyover the exterior surface of the welded seam 70. As described above,this apparatus is preferably enclosed within an enclosure which isfilled with nitrogen gas, preventing oxidation of the weld and themolten zinc, assuring a good adhering coating over the weld. Acylindrical plug 180 is connected by tether 182 to the following end ofthe manifold 162. The plug 180 limits the escape of nitrogen gas tomaintain the inner surface of the tube in an inert atmosphere.

FIG. 11 illustrates a further means of directing the flow of molten zincto the welded seam. As shown in FIG. 11, the arcuate strip 20a isreceived between rollers 184 and 186. The inner roller 186 includes aherring bone pattern which imparts a herring bone pattern 188 to theinner surface of the strip on opposite sides of the seam. The rollers184 and 186 may be located at the beginning of the forming station 58 orin the edge conditioning station 54. When the tube is rolled into anopen seam tube 20b as shown in FIG. 4, the herring bone pattern will belocated adjacent the inner edges 48 of the open seam tube. The herringbone pattern will thus direct the molten zinc toward the seam, improvingthe flow of zinc over the seam.

FIGS. 14-17 illustrate a preferred embodiment of an impeder 200 whichmay be utilized with the tube forming and coating process of thisinvention. The impeder 200 includes an outer casing 202 formed of anonmetallic, nonconductive magnetically permeable material. Inside theouter shell 202 is a first outer annular chamber 204 which containsferrite rods 206 as best shown in FIG. 15. The ferrite rods 206 arepreferably located in close proximity to the outer casing 202. In thedisclosed embodiment of the impeder, the ferrite rods 206 are arrangedin a ring and radially spaced adjacent the shell 202. The ferrite rodsare supported in a cradle 208 formed of a nonconductive metallicmaterial. As shown in FIG. 15, the cradle, which may be formed of a heatresistant thermoset plastic, further divides the outer chamber 204 intoan outer portion 208 and an inner portion 210. Liquid coolant, which maybe water or a light oil, is circulated around the ferrite rods byinjecting the coolant under pressure into one port 212 in the bracket orsupport portion 214 into the outer chamber 208. The coolant thencirculates through the free end 2 16 of the casing, where it returnsthrough the inner portion 2 10 of the chamber 204 and out the outletport 218 in the bracket or support portion 214. Coolant removes heatfrom the ferrite rods which are heated during the induction welding ofthe tube. An inert gas, preferably nitrogen, is injected under pressureinto the inner ports 220 in the bracket portion. The inert gas is thenreceived in chamber 220 at the end of tube 224. The axial tube 224extends through the free end 216 of the shell 202 and floods the insideof the tube with inert gas, as described above. The coolant inlet port212 is connected by line 226 to a coolant tank 228. A conventional pump230 pumps the coolant through the impeder, as described. A return line232 returns the coolant to tank 228.

FIG. 18 illustrates one preferred embodiment of a lateral edge 48 of thestrip 20 following edge conditioning at station 54 in FIG. 1. As shown,the metal coating 56, which is preferably zinc, has been removed fromthe edges 48 as shown in phantom to expose the edges of the metal strip20, which is preferably steel. In the preferred embodiment of thelateral edge 48 shown in FIG. 18, the inner edge 300 is preferably cutat an angle alpha, as shown, removing a triangular portion 302 of themetal strip and the zinc coating 56A. The end or edge 304 is cut squareto the strip and the outer surface 306 is exposed, as shown. As usedherein, "inner" refers to the surface of the strip which forms the innersurface of the tube and "outer" refers to the surface of the strip whichforms the outer surface of the tube.

In the disclosed embodiments, the metal coating covering the inner andouter surfaces of the strip 20 is preferably removed to expose or barethe steel strip adjacent the edges 304, such that less of the metalcoating 56 is burned-off or vaporized during welding. The ends 304 arepreferably square and exposed to assure a good weld. Finally, the innersurfaces 300 adjacent the ends 304 are preferably formed to define aninner concave depression or well in the tube which receives the metalcoating as the molten coating metal flows downwardly over the weld orseam inside the tube, as described below. As described above, the strip20 may be fiat or preformed into an arcuate shape 20a as shown in FIG.2.

FIG. 19 illustrates an alternative preferred embodiment of the lateraledge 448 of the strip 20. As described above in regard to FIG. 18, themetal coating 456A covering the inner and outer surfaces of the strip 20are preferably removed to bare the steel strip adjacent the ends 404,such that less of the coating 456 is burned-off or vaporized duringwelding. In the embodiment shown in FIG. 19, however, the metal coating456 increases in thickness toward the edges 448, which is shown somewhatexaggerated in FIG. 19. As described above, this provides more coatingmetal to reflow over the welded seam in the process of this invention.The desired overall thickness of the metal coating is shown in phantomat 456B. A portion 402 is removed from the steel strip at the edgeconditioning station 54 (FIG. 1), such that the exposed surface 400forms a well or concave depression adjacent the welded seam whichreceives molten coating metal as described above. The ends 404 arepreferably relatively flat and generally transverse to the inner andouter side surfaces of the strip 20 to assure a good weld and the outersurface 406 is exposed to limit vaporization of the metal coating asdescribed above.

FIG. 20 illustrates the tube 20c adjacent the seam 71 following welding.The embodiment of the seamed tube 20c shown in FIG. 20 was formed from astrip having chamfered or inwardly angled surfaces 300 adjacent thewelded seam 71 as described above in regard to FIG. 20. As will beunderstood by those skilled in the art, the forging of the molten edges304 by the squeeze rolls 68 (see FIGS. 1 and 8) results in upset flasheson the inner and outer surfaces of the tube. The outer flash 312 isremoved by a scarfing tool 70; however, the inside flash 310 cannot bescarfed or removed by conventional means. As described above, the heightof the inside flash 310 can be reduced by chamfering the inside surfaceadjacent the lateral edge as shown at 300 in FIG. 18 or by providing areduced thickness portion as shown at 400 in FIG. 19. Further, thechamfered surface 300 guides the molten metal of the coating 56downwardly to the upset portion 310 of the seam 71 and provides a wellreceiving the molten metal.

FIG. 21 is a cross-sectioned end view of a portion of the tube 20cfollowing reflow of the metal coating, preferably zinc, over the weldedseam 71. As shown, the coating 56 flows downwardly over the opposedinwardly inclined surfaces 300 on the inside of the tube and accumulatesin the recess or well coating the seam, as shown at 56C in FIG. 21.Similarly, the zinc coating flows downwardly over the outside surface ofthe tube coating the outer surface of the seam, as shown at 56D. If aherring bone pattern is formed on the inner surface of the tube, asdescribed above in regard to FIG. 11, the herring bone pattern should beapplied on the chamfeted surface 300 after edge conditioning. Asdescribed above, the tube may be reheated above the melting temperatureof the zinc or other metal coating to cause the coating to flowdownwardly over the seam as shown in FIG. 21 and the molten metal may bedriven downwardly by inert gas pressure as shown, for example, at FIG.12.

As will now be understood, the forming and welding of the tubeupside-down following galvanizing with the seam located in the bottom ofthe tube provides for the coating of the inner and outer surfaces of theseam without acquiring special coating equipment. There are disclosedherein several means for driving and directing the flow of molten zincover the seam, including reheating the tube, driving the molten zincwith an inert gas and forming flow paths in the base metal. These meanscan be used alone or in combination, as required by the parameters ofthe particular application, including tube size, mill speed, tube andcoating material, welding temperature, etc. Further, variousmodifications may be made to the tube forming and coating process andapparatus of this invention within the purview of the appended claims.Further, mix patterns may be applied to the inner and outer surfaces ofthe tube adjacent the seam to direct the flow of molten metal into theseam in addition to or instead of the herring bone pattern shown in FIG.11. Further, the internal gas manifold 162 of FIG. 12 may be usedwithout the external gas system shown in FIG. 12. The tube may also bereheated by any suitable heating means, including radiant heaters.Finally, the tube may be welded by other conventional means, including,for example, resistance or flux welding.

We claim:
 1. A method of forming a seamed metal tube having a metalcoating from a continuously moving relatively flat metal strip,comprising the following steps performed in sequence:a) applying acoating to at least one side surface of said strip with a metal coatingselected from the group consisting of zinc, aluminum and their alloys;b) rolling and forming said strip into a tube-shaped strip havingopposed adjacent spaced lateral edges in a lower portion of saidtube-shaped strip; c) heating and integrally welding said adjacent edgesof said strip to form a tube having a welded seam in said lower portionof said tube; and d) reheating at least said lower portion of said tubeto the melting temperature of said metal coating, said metal coatingthen flowing downwardly over and coating said seam.
 2. The method offorming a coated seamed tube as defined in claim 1, wherein said methodincludes coating said strip with a metal coating having a thicknesswhich increases from a mid-portion toward said lateral edges.
 3. Themethod of forming a coated seamed tube as defined in claim 1, whereinsaid method includes welding said tube edges and reheating said tube ina non-oxidizing atmosphere.
 4. The method of forming a coated seamedtube as defined in claim 3, wherein said method includes heating andwelding said edges of said tube-shaped strip and reheating said tube inan enclosure, said method further including continuously introducing aninert gas under pressure into said enclosure maintaining saidnon-oxidizing atmosphere.
 5. The method of forming a coated seamed tubeas defined in claim 4, wherein said method includes cooling said tubefollowing reheating.
 6. A method of forming a seamed metal tube having ametal coating from a metal strip, comprising the following stepsperformed in sequence:a) coating said strip with a metal coating spacedfrom the lateral edges of said strip; b) rolling and forming said stripinto a tube-shaped strip having opposed adjacent spaced lateral edges;c) inductively heating at least said opposed lateral edges of said stripby moving said strip continuously through an induction heating coil withsaid opposed adjacent spaced lateral edges oriented generally downwardlyand forging said edges together to continuously form an integrallyseamed tube having a welded seam oriented generally downwardly; and d)reheating at least said lower portion of said tube to the meltingtemperature of said metal coating, said metal coating then flowingdownwardly over and coating said seam.
 7. The method of forming a coatedseamed tube as defined in claim 6, wherein said method includes coatingsaid strip with a metal coating which increases in thickness from amid-portion toward said lateral edges;
 8. The method of forming a coatedseamed tube as defined in claim 6, wherein said method includesinductively heating and forging said tube lateral edges and reheatingsaid tube in a non-oxidizing atmosphere.
 9. The method of forming acoated seamed tube as defined in claim 8, wherein said method includesinductively heating and forging said edges of said tube-shaped strip andreheating said tube in an enclosure, said method further includingcontinuously introducing nitrogen gas under pressure into said enclosuremaintaining said non-oxidizing atmosphere.
 10. A method of forming aseamed metal tube having an internal and external metal coating from acontinuously moving relatively flat metal strip, comprising thefollowing steps performed in sequence:a) applying a coating to at leastone side surface of said strip with a metal coating; b) continuouslyrolling and forming said strip into a tube-shaped strip having opposedadjacent spaced lateral edges in a lower portion of said tube-shapedstrip; c) heating and continuously forging said opposed adjacent spacedlateral edges of said tube-shaped strip in a sealed enclosure to form atube having a welded seam in said lower portion of said tube; and d)introducing an .[.inert.]. .Iadd.non-oxidizing .Iaddend.gas underpressure into said enclosure forming a non-oxidizing atmosphere in saidenclosure with the temperature of said tube adjacent to and includingsaid seam above the melting temperature of said metal coating and saidmetal coating flowing downwardly and coating said seam followingforging.
 11. The method of forming a seamed metal tube as defined inclaim 10, wherein said method includes directing a jet of inert gasunder pressure over a surface of said tube toward said seam forging ofsaid edges, said inert gas directing said flow of molten metal coatingover and coating said seam.
 12. The method of forming a seamed metaltube as defined in claim 10, wherein said method includes directing aplurality of jets of inert gas under pressure over an inner surface ofsaid tube, adjacent said seam generally towards said seam, wherein saidstep of directing said plurality of jets over an inner surface of saidtube follows forging of said tube lateral edges, said inert gas jetdirecting said flow of molten metal coating over and coating saidinternal seam.
 13. The method of forming a seamed metal tube as definedin claim 10, wherein said method includes reheating at least said lowerportion of said tube to above the melting temperature of said metalcoating within said enclosure, said metal coating then flowingdownwardly over and coating said seam.
 14. The method of forming aseamed metal tube as defined in claim 13, wherein said method includescooling said tube following reheating.
 15. The method of forming aseamed metal tube as defined in claim 13, wherein said method includesdirecting a jet of inert gas under pressure over a surface of said tubetowards said seam following reheating of said tube, said inert gas jetdirecting said flow of molten metal coating over and coating said seam.16. The method of forming a seamed metal tube as defined in claim 13,wherein said method includes directing a plurality of jets of inert gasunder pressure over an internal surface of said tube adjacent said seamand towards said seam following reheating, said inert gas jet directingsaid flow of molten metal coating over and coating said seam innersurface.
 17. The method of forming a seamed metal tube as defined inclaim 13, wherein said method includes scarfing said seam within saidenclosure before reheating.
 18. The method of forming a seamed metaltube as defined in claim 17, wherein said method includes scarfing saidseam with a scarfing knife located in a tube extending out of saidenclosure and said method including introducing nitrogen under pressureinto said tube.
 19. A method of forming a seamed metal tube having aprotective metal coating from a continuously metal strip, comprising thefollowing steps performed in sequence:a) applying a coating to at leastone side surface of said strip with a metal coating, said metal coatinghaving a melting temperature substantially below the melting temperatureof said strip; b) conditioning the lateral edges of said strip byremoving portions of strip at said lateral edges and forming concaveindented surfaces adjacent each of said lateral edges in one surface ofsaid strip; c) rolling and forming said strip into an open seam annulartube having said concave indented surfaces on an inside lower surface ofsaid open seam tube with said lateral edges nearly abutting in a lowerportion of said open seam tube; d) heating said open seam tube adjacentsaid lateral edges to a temperature above said metal coating meltingtemperature and integrally welding said nearly abutting lateral edges toform a seam in said lower portion of said tube, and melting and flowingsaid metal coating downwardly into said concave indented surfaces andcoating said seam.
 20. The method of forming a seamed metal tube asdefined in claim 19, wherein said method includes forming generallyrectangular concave indented surfaces opening through said lateral edgesin said one surface of said strip to define said concave indentedsurfaces, then rolling and forming said strip into an annular tubehaving nearly abutting lateral edges in a lower portion of said tubewith said generally rectangular concave indented surfaces creating anddefining a rectangular well in said lower portion of said open seamedtube, then heating and integrally welding said adjacent edges of saidstrip to form a tube having a welded seam, then reheating at least saidlower portion of said tube to said metal coating melting temperature,said metal coating then melting and flowing downwardly into said concavewell, coating said seam.
 21. The method of forming a seamed metal tubeas defined in claim 19, wherein said method includes removing said metalcoating from opposed surfaces of said strip adjacent said lateral edgesprior to rolling and forming said strip.
 22. The method of forming aseamed metal tube as defined in claim 19, wherein said method includesforming chamfered inclined edges on said one surface of said strip atsaid lateral edges, said chamfered edges defining said concave indentedsurfaces, then rolling and forming said strip into a generally circulartube having nearly abutting lateral edges in a lower portion of saidtube and said chamfered surfaces defining inwardly angled surfaces atsaid lateral edges, then heating and integrally welding said lateraledges of said strip to form a tube having a welded seam in said lowerportion of said tube with said chamfered edges defining inwardlyinclined surfaces at said seam, said metal coating melting and flowingdownwardly on said inclined surfaces over said seam.
 23. The method offorming a seamed metal tube as defined in claim 22, wherein said methodincludes reheating at least said lower portion of said tube to saidmetal coating melting temperature following welding of said nearlyabutting edges of said strip, said metal coating then melting andflowing downwardly over said inwardly inclined surfaces and coating saidseam.
 24. A method of forming a seamed metal tube having a metal coatingfrom a continuously relatively flat metal strip, comprising thefollowing steps performed in sequence:a) applying a coating to at leastone side surface of said strip with a metal coating having a meltingtemperature substantially below the melting temperature of said strip byimmersing said strip in a molten metal; b) removing said metal coatingfrom the lateral edges of said strip and forming depressed surfaces onone surface of said strip at said lateral edges; c) rolling and formingsaid strip into an open seam annular tube having said depressed surfaceson an inside lower surface of said open seam tube with said lateraledges nearly abutting in a lower portion of said open seam tube and saiddepressed surfaces forming an open concave indented surface adjacentsaid lateral edges; d) heating said open seamed tube adjacent saidlateral edges to a temperature above said metal coating meltingtemperature and integrally welding said nearly abutting lateral edges,forming a seam in said lower portion of said tube, and then melting andflowing said metal coating downwardly into said concave indented surfaceand coating said seam.
 25. The method of coating a seamed metal tube asdefined in claim 24, wherein said method includes removing said metalcoating on opposed surfaces of said strip adjacent said lateral edgesprior to rolling and forming said strip.
 26. The method of forming aseamed metal tube as defined in claim 25, wherein said method includesforming chamfered inclined edges on said one surface of said strip atsaid lateral edges, then rolling and forming said strip into a generallycircular tube having nearly abutting lateral edges in a lower portion ofsaid tube, said chamfered inclined surfaces then defining opposedinwardly inclined surfaces at said nearly abutting lateral edges of saidopen seamed tube, then heating said open seamed tube adjacent saidlateral edges and welding said lateral edges, said metal coating thenmelting and flowing downwardly over said opposed inwardly inclinedsurfaces and coating said seam.
 27. The method of forming a seamed metaltube as defined in claim 26, wherein said method includes reheating atleast said lower portion of said tube to a temperature above said metalcoating melting temperature following welding of said lateral edges,said metal coating then melting and flowing downwardly over said opposedinclined surfaces coating said seam.
 28. The method of forming a seamedmetal tube as defined in claim 24, wherein said method includes forminggenerally rectangular depressed surfaces on said one surface of saidstrip opening through said lateral edges, said depressed surfacesforming a generally rectangular well on opposed sides of said seamfollowing welding and said molten metal coating flowing downwardly intosaid generally rectangular well and coating said seam.
 29. The method offorming a seamed metal tube as defined in claim 28, wherein said methodincludes reheating at least said lower portion of said tube to saidmetal coating melting temperature following welding, said metal coatingthen melting and flowing downwardly to said generally rectangular well,coating said tube.
 30. A method of forming a seamed metal tube having ametal coating from a continuous metal strip, comprising the followingsteps performed in sequence:a) applying a coating to at least one sidesurface of said strip with a protective metal coating, said metalcoating having a melting temperature substantially below the meltingtemperature of said strip, by immersing said strip in a molten metal; b)removing said metal coating from the lateral edges of said strip and theopposed surfaces of said strip adjacent said lateral edges and formingdepressed surfaces on one surface of said strip at said least lateraledges; c) rolling and forming said strip into an open seamed annulartube having said depressed surfaces on an inside surface of said openseamed tube with said lateral edges nearly abutting in a lower portionof said open seamed tube and said depressed surfaces forming an openconcave indented surface at said lateral edges; d) heating andintegrally welding said lateral edges of said strip to form a tubehaving a welded seam in said lower portion of said tube with said openconcave indented surface on opposed sides of said seam; and e) reheatingat least said lower portion of said tube to said metal coating meltingtemperature, said metal coating then melting and flowing downwardly intosaid open concave indented surface and coating said seam.
 31. The methodof forming a seamed metal tube as defined in claim 30, wherein saidmethod includes forming chamfered inclined surfaces on one surface ofsaid strip at said lateral edges, then rolling and forming said stripinto a generally circular tube having nearly abutting lateral edges in alower portion of said tube with said chamfered surfaces defining opposedinwardly inclined surfaces on opposed sides of said seam followingwelding.
 32. The method of forming a seamed metal tube as defined inclaim 30, wherein said method includes forming generally rectangularindented surfaces opening through said lateral edges on said one surfaceof said strip, said generally rectangular surfaces defining a generallyrectangular well on opposed sides of said seam following welding, saidmethod including reheating at least said lower portion of said tube,said metal coating then flowing downwardly into said generallyrectangular well and coating said seam.